<form role="search" method="get" action="<?php bloginfo('url'); ?>">
  <div style="clear: both;"></div>
  <input class="form-control" type="text" placeholder="Search" name="s" style="float:left; width:73%;" />
  <input class="btn btn-default btn-primary" type="submit" value="搜索" style="float:right;width:20%;" />
  <div style="clear: both;"></div>
</form>